Create a circle 8 units in radius, mirror it and project to lower surface.
Split and move the circles down by 2 units.
Next, either copy the circle, and move it also by 2 units down, or you can using DupEdge create a circle out of edge of the circle surface. Offset that circle by 0.5 units inside, and use it as a reference for creating a sphere.
Trim off the upper part of a sphere, and loft the edges of the big surface and little circle surface. After that copy that part on the other side too, and join everything.